1. Field of the Invention
The present invention relates to a moisture sensor comprising a substrate, an electrode and a moisture sensitive polymer film. More particularly, it relates to a moisture sensor wherein the moisture sensitive polymer film is tightly bound to the substrate to ensure a stable and reproducible moisture sensitive characteristic.
2. Description of the Prior Art
It has been known that a moisture sensor having a polymer film formed on a substrate can detect humidity in the atmosphere due to variation of the electrostatic capacity of the polymer film. However, the polymer film materials have little affinity for the substrate resulting in the lack of adhesion to the substrate, so that an accurate electric signal is not obtainable from such a conventional moisture sensor.
FIG. 1 shows a structure of a typical moisture sensor having an organic polymer film, wherein on a substrate 1 made of an insulator such as a metal oxide, glass or the like, a moisture sensitive film 2 made of an organic polymer is formed and a moisture permeable electrode 3 is further formed on the moisture sensitive polymer film 2. The moisture sensor detects variation of humidity in the atmosphere as variation of an impedance depending on variation of the electrostatic capacity of the organic polymer film.
The organic polymer film is generally formed on the substrate in such a manner that an organic polymer solution which is prepared by dissolving an organic polymer in a solvent is coated on the substrate by a spin casting technique or an immersion technique followed by a baking treatment or a drying treatment to form a moisture sensitive film. Alternatively, the moisture sensitive film is directly formed on the substrate or the electrode by a spattering technique or a plasma CVD technique. However, the resulting moisture sensitive film has insufficient adhesion to the substrate, resulting in an unstable impedance and/or an apparent aged deterioration of the moisture sensitive characteristic (humidity/impedance characteristic) thereof. This is most easily observed when the moisture sensitive film is formed by a spin casting technique or an immersion technique. It is presumed that the moisture sensitive film materials do not have strong chemical bonding strength to the substrate materials and that a swelling and/or a shrinkage occurs in the moisture sensitive film depending upon the amount of water absorbed in the film, namely humidity in the atmosphere, thereby further weakening the adhesion between the film and the substrate.